The University of Texas M.D. Anderson Cancer Center said Friday a portable USB thumb drive containing some patient and research data was lost on July 13.

The unencrypted thumb drive was lost on an M.D. Anderson employee shuttle bus, and M.D. Anderson began a search for the device on July 14, immediately after learning about the incident, the cancer center said in a statement.

M.D. Anderson on Friday began mailing letters to approximately 2,200 patients whose information was on the drive.

The cancer center said it does not believe the information has been or will be accessed improperly. The drive included patient names, dates of birth, medical record numbers and diagnoses, and treatment and research information, but it did not contain Social Security numbers or other financial information.

M.D. Anderson said in the statement that it “deeply regrets that this incident has occurred.” No one was available for comment Friday afternoon.